THE DAILY MARCH OF TRANSPIRATION IN A DESERT 



PERENNIAL. 



INTRODUCTION. 



A consideration of the various types of perennial plants indigenous to the 

 vicinity of the Desert Laboratory at Tucson, Arizona, brings to view some 

 striking differences between the species concerned, in respect to their total 

 annual water-losses. The perennials fall into the general types, succulent 

 and non-succulent, the latter being easily classified further into three physio- 

 logical groups, namely: (1) small plants whose perennial parts are confined 

 to roots or underground stems; (2) plants continuously in leaf; (3) tropo- 

 phytic plants. Examples of the second class are Covillea iridentata, Lijcium 

 herlandieri , Celtis crassifolia, Encelia farinosa, Hypiis emoryi; examples of 

 the third class are Prosopis velutina, Acacia greggii, Fouquieria splendens, 

 Jatropha cardiophylla, and Parkinsonia microphylla. The tropophytic 

 plants are alike in that they all drop their leaves more or less quickly in 

 times of drought, but ParA-msowia microphylla diEevs, from the others in hav- 

 ing a continuous covering of epidermis over all of its parts, even the limbs 

 over 100 years old having an active chlorphyll layer covered with an 

 unbroken living epidermis. Thus, although the trees are without leaves 

 during times of drought, they must still lose a large quantity of water. 

 Some of the evergreen plants have a continuous epidermis, but these are all 

 much smaller than Parkinsonia and hence expose a smaller evaporating 

 surface. In spite of this necessity for a large loss of water, Spalding* 

 calls Parkinsonia a "highly successful desert species, growing in abundance 

 and equally well on slopes of all exposures." Shrevef found a high death- 

 rate in seedlings of Parkinsonia, but reached the conclusion that the critical 

 period in the life of the plant is over after the first two years. 



The abundance of Parkinsonia on hill slopes in an arid region is an index of 

 its apparent success in spite of the exposure of a large evaporating surface 

 during the entire year and the difficulty its seedlings have during the first 

 two years. In consequence of the ability of Parkinsonia to overcome 

 these adverse conditions, it was selected as the subject of transpiration- 

 behavior studies. A general study of transpiration behavior of seedlings and 

 adult plants during different seasons of the year is in progress, but certain 

 fluctuations in the daily transpiration rate make a separate publication on 

 this phase of the work advisable. Only such description of the plant, its 

 habitat, and its anatomical features will be given as seems to apply either 

 directly or indirectly to the phase of transpiration mentioned. 



*Spalding, V. M., Distribution and movements of desert plants, Cam. Inst. Wash. 

 Pub. 113, 1909. 



tShreve, F., Establishment behavior of palo verde, Plant World, vol. xiv, p. 294, 1911. 



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